Hexagonal hybrid bismuthene by molecular interface engineering

Abstract text (incl. figure legends and references)

Since the rise of graphene as playground for scientists in different fields, the fascination for two-dimensional materials did not cool down but rather remains at a peak interest due to the exceptional properties those ultrathin, layered structures show. While physicists are intrigued by transport properties for TMD's and material scientists praise the mechanical stabilities of even single layer thick atomic layers, chemists mainly use 2D materials as perfectly defined substrate for functionalization approaches.

In contrast to widely studied materials, we will present a chemical approach towards two-dimensional bismuth with unprecedented material quality and a surprisingly unexpected structure. In an one-pot colloidal reductive reaction, we synthesized single crystalline hexagonal platelets with diameters up to 2 µm and a thickness around 5 nm. Due to the chemical approach for the generation of these ultrathin bismuth material, we were able to introduce structurally modified bismuthene on the basal planes of the crystallites that are stabilized by a covalent functionalization.

To reveal the nature and resulting properties of this sandwiched hybrid structure we made use of a battery of characterization techniques ranging from microscopy (HRTEM, HRSTEM, EELS, EDS, SAED, AFM) to spectroscopy (XPS, Raman) and finally simulations (DFT) and single-particle transport measurements. The combination of all these techniques allowed us to describe the stabilized bismuthene capping layers as fundamentally different from pristine rhombohedral bismuth. Firstly, while bismuth shows the typical zig-zag arrangement with a lattice constant of 4.5 Å, the bismuthene layers are flat with a lattice parameter of 7.5 Å. Monochromated STEM-EELS reveals LSPR modes that are highly unexpected for a semimetal like bismuth and the calculated band structure, based on the simulated structure, shows metallic behaviour. The metallic nature of individual crystals was measured by transport experiments at room temperature.

In this contribution we show the wholistic process of an exceptional new material with a strong spin orbit coupling and postulated spin torque effects from the bottom-up synthesis to the characterization and conceivable future applications in devices.

Fig. 1 A Colloidal synthesis of the sandwich material with equivalents for the reaction. B PXRD pattern of synthesized material with expected diffraction peaks indicated. C Typical unprocessed Raman spectrum. No indication of surface oxidation. Inset optical micrograph of crystallite. D BF-TEM data and E typical indexed diffraction pattern of hybrid material with superstructural reflections indicated. F Colorized large area HRSTEM data with FFT as inset. Superstructure indicated in FFT. G AFM with height profile. H Experimental series of integrated EELS intensity, showing localized surface plasmon modes and I simulation of plasmonic modes based on model hexagon.

Fig. 2 A Integrated STEM-EDX intensity for individual crystallite with artistic representation of functionalized material. B Spatially integrated mean EDX spectrum with signals related to Bi. C Overview XPS before (red) and after (black) surface etching with Ar ions and D high resolution XPS of S/ Bi binding energies. A clear indication of S 2p before the surface etching can be noticed between 165–170 eV. E Structurally optimized bismuth/ bismuthene hybrid with simulated ED. Bottom right cross sectional HRTEM and respective image simulation.